Flow Boiling Heat Transfer Characteristics of R134a in a Horizontal

Jul 28, 2009 - ... heat transfer characteristics of 1,1,1,2-tetrafluoroethane (R134a) were experimentally investigated in a horizontal stainless steel...
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2638

J. Chem. Eng. Data 2009, 54, 2638–2645

Flow Boiling Heat Transfer Characteristics of R134a in a Horizontal Mini Tube† Lihong Wang,‡ Min Chen,*,‡ and Manfred Groll§ Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China, and Institute of Nuclear Energy & Energy Systems, University of Stuttgart, Stuttgart 70569, Germany

Flow boiling heat transfer characteristics of 1,1,1,2-tetrafluoroethane (R134a) were experimentally investigated in a horizontal stainless steel mini tube. The inner diameter of the test tube is 1.3 mm, and the tube wall thickness is 0.1 mm. Local heat transfer coefficients are obtained over a range of vapor qualities up to 0.8, mass fluxes from (310 to 860) kg · m-2 · s-1, heat fluxes from (21 to 50) kW · m-2, and saturation pressures from (0.65 to 0.75) MPa. The dependences of heat transfer coefficients on mass flux, heat flux, saturation pressure, and vapor quality are demonstrated. On the basis of an available model in recent literature, potential heat transfer mechanisms are also analyzed.

1. Introduction Over the past decades, compact heat exchangers, because of their remarkable advantages such as higher heat transfer area density, small size, and better heat transfer characteristics, are gaining increasing attention.1-3 Extensive applications exist in industries where more compact heat exchangers with a twophase operation may provide higher performance and better flexibility to remove heat than those employing a single-phase operation, such as the potential applications in thermal control of spacecraft payloads and other aerospace areas.4 Meanwhile, microscale heat exchangers are becoming necessary in many microelectronic mechanical systems (MEMS), which are very sensitive to temperature. In recent years, a number of researchers have carried out experiments on flow boiling heat transfer in mini-size tubes and channels. An experiment on flow boiling heat transfer of refrigerant 1,1,2-trichlorotrifluoroethane (R113) in a horizontal tube with an inner diameter of 2.92 mm was conducted by Wambsganss et al.5 They reported that, for saturated boiling, the heat transfer coefficient was strongly dependent on the heat flux and hardly dependent on the vapor quality. Oh et al.6 studied the saturated flow boiling heat transfer for 1,1,1,2-tetrafluoroethane (R134a) in horizontal tubes with inner diameters of 0.75 mm, 1 mm, and 2 mm. They observed that the heat transfer coefficient was a strong function of vapor quality. In the low quality region, the measured heat transfer coefficient increased rather slowly with quality, whereas in the higher quality region there was a relatively rapid increase in the heat transfer coefficient when quality increased. The local flow boiling heat transfer of 1,1-dichloro-1-fluoroethane (R141b) was investigated in horizontal tubes with diameters of (1.39 to 3.69) mm by Kew and Cornwell.3 Their experiments demonstrated that the heat transfer coefficient had a strong dependence upon vapor quality in the higher quality region and there was negligible influence of vapor quality in the lower quality region. The independently measured heat transfer coefficients by Zhang et al.7 and Bertsch et al.8 also drew the same conclusion. An experiment on flow † Part of the “William A. Wakeham Festschrift”. * Corresponding author. E-mail: [email protected]. Tel.: +86 1062773776. Fax: +86 1062795832. ‡ Tsinghua University. § University of Stuttgart.

boiling was conducted by Lin et al.,9 with R141b in a tube with diameter of 1 mm. They found that under low heat flux the heat transfer coefficient was almost constant with increasing quality while under high heat flux the heat transfer coefficient sharply decreased with increasing quality in the whole saturated boiling region. Huo et al.10 performed a flow boiling experiment in a small tube with R134a and obtained a similar result as that of Lin et al.9 Recently, Shiferaw et al.11 carried out flow boiling experiments and compared their results with a three-zone evaporation model developed by Thome et al. The comparison indicated that the flow boiling heat transfer coefficient decreased with the vapor quality increase, especially at low pressures. It is noticeable that the boiling heat transfer characteristics obtained so far for mini-size channels are in poor agreement or even in contradiction to each other. Our understanding of the physical mechanisms behind those experiments is not sufficient to allow a deep and comprehensive analysis. Consequently, the experiments described in the following are carried out as part of a project aimed to clarify the flow boiling heat transfer mechanisms in small channels. Some contradictions about boiling heat transfer mechanisms in small channels will be reconciled, in part, by the results of this study.

2. Experimental Apparatus and Procedures The experimental apparatus is schematically shown in Figure 1, which consists of four main systems, namely, the refrigerant loop, the cooling water loop, the direct current (DC) power supply for heating the test section, and a data acquisition system. Refrigerant R134a is circulated in the refrigerant loop. The cooling water loop has enough cooling capacity to remove the heat generated by the DC power supply and regulate the inlet temperature of the test section. The refrigerant loop contains a reservoir, a magnetic pump, a volume flow meter, a test section, and two condensers. The working fluid is pumped in the refrigerant loop by a magnetic pump with rated output. By adjusting the fine valve and coarse valve simultaneously, the flow rate can be regulated. The excess fluid flows back to the reservoir through the coarse valve while the wanted part flows through the fine valve, fine condenser, flow meter, test section, coarse condenser, and reservoir in sequence, thus completing a circulation. The coarse condenser with larger cooling capacity is used to condense the vapor from

10.1021/je900140w CCC: $40.75  2009 American Chemical Society Published on Web 07/28/2009

Journal of Chemical & Engineering Data, Vol. 54, No. 9, 2009 2639

segments keep the difference in temperature between the inside and the outside of the second layer of insulating thermal material as small as possible, which insures that the outside surface boundary condition is as close as thermal insulated. All experimental data are collected by a data acquisition system with an uncertainty of ( 0.5 %. The thermophysical properties of the working fluid were taken from the ASHRAE Handbook Fundamentals 2005.12

3. Data Reduction

Figure 1. Schematic diagram of the experimental system: 1, reservoir; 2, magnetic pump; 3, fine valve; 4, fine condenser; 5, flow meter, 6, test section; 7, coarse condenser; 8, coarse valve; 9, sight glass; 10, DC power supply.

Prior to performing flow boiling heat transfer experiments, single-phase heat transfer experiments are carried out to check the experimental system and to calibrate the heat loss. The results demonstrate that the system is reliable and the heat loss is less than 1.5 %. Considering the small tube wall thickness (0.1 mm), axial heat conduction will not be taken into account. The local heat transfer coefficient, at position z along the length of the tube, is defined as eq 1.

h(z) )

q Twi(z) - Tsat(z)

(1)

where h is the heat transfer coefficient and the inner wall temperature of the tube Twi(z) is obtained from eq 2 by taking the tube outside surface as insulated.

Figure 2. Schematic diagram of the test section: 1, stainless steel tube; 2, thermocouples; 3, copper sleeves of DC power supply; 4, electrical resistance; 5, AC power supply.

the test section. At the same time, the fine condenser controls the inlet temperature of the working fluid. The flow rate is measured by the volume flow meter by McMillan Company with an accuracy of 0.8 % in the range of (13 to 100) mL · min-1. In addition, pressures at inlet and outlet are measured by a pressure difference transducer with an uncertainty of 0.1 %, in the range of (0 to 1.2) MPa. As schematically shown in Figure 2, the test section is a horizontal stainless steel tube with an inner diameter (Dh) of 1.3 mm and a thickness (δ) of 0.1 mm. The heated length (L0) of the test section is 720 mm. To reach the high heat flux working condition, a subtest section with a heated length (l) of 480 mm is adopted. DC electrical power is supplied to the stainless steel tube sections via copper sleeves to obtain a uniform heat flux boundary condition. The copper sleeves are silver brazed to the ends of each tube section and have been sized to produce negligible heat generation compared with the heat generated in the stainless tube. The voltage drop across the test section is measured directly across the copper sleeves, whereas the current is determined by putting a calibrated resistance in series with the test section and measuring the voltage across it. Outer wall temperatures of the test section and the subtest section are measured at 17 and 13 axial locations along the length of the tube, respectively, by copper-constantan thermocouples with a calibrated uncertainty of 0.05 °C. At each location, two thermocouples are symmetrically located at the top and the bottom of the tube. All thermocouples are pasted to the outside surface of the tube with electrically insulating glue. Four layers of thermal-insulating material and three segments of thermal guards are wrapped around the outside surface of the test section to reduce the heat loss. Each of the three segments of thermal guards consists of an electrical resistance, an alternating current (AC) power supply, and a temperature-difference feedback system. The three thermal guard

Twi(z) )

[

1q r2 ks

() r r0

2

() ()

- 2 ln 1-

r -1 r0

r r0

2

]

+ Two(z) (2)

In eq 1, linear interpolation is applied to determine the fluid saturation pressures at each measurement location along the test tube, after which the fluid saturation temperatures Tsat(z) are determined. Tran et al.13 verified that an error was hardly introduced using the linear pressure drop. The length of the subcooled inlet region is calculated iteratively

Losb

osb GDhCp(Tsat - Tin) ) 4q

(3)

where Cp is the specific heat capacity at constant pressure, Dh the tube hydraulic diameter, G the mass flux, and q the heat flux. The local vapor quality at positions x(z) is determined by a thermal balance from the equation

x(z) )

qπDh(z - Losb) hfgAG

(4)

where A is the tube flow area, L the length of the test section, and hfg the specific enthalpy of vaporization. The process of uncertainty analysis described by Moffat14 is used to estimate the experimental uncertainties in this study. On the basis of the calibrations of the entire temperature measurements, including thermocouples and the data acquisition system, the uncertainties associated with the temperature and temperature difference data are ( 0.19 °C and ( 0.27 °C, respectively. The uncertainty in heat flux, which depends on the measurements of the voltage and the current across the test section, is 0.24 %. The uncertainty in the refrigerant mass flux is 1.8 %. Consequently, the experimental uncertainty in the heat transfer coefficient is estimated to be under ( 12.0 % for low heat flux measurements and about ( 10.3 % for a moderate mass and heat flux run.

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Journal of Chemical & Engineering Data, Vol. 54, No. 9, 2009

Table 1. Measured Local Heat Transfer Coefficient h under Average Saturation Pressure p, Heat Flux q, Mass Flux G, and Vapor Quality x p

q

G

p

q

G

no.

MPa

kW · m-2

kg · m-2 · s-1

x

kW · m-2 · K-1

h no.

MPa

kW · m-2

kg · m-2 · s-1

x

kW · m-2 · K-1

h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

0.651 0.651 0.651 0.651 0.651 0.651 0.651 0.651 0.651 0.651 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675

50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 42.9 42.9 42.9 42.9 42.9 42.9 42.9 42.9 42.9

836 836 836 836 836 836 836 836 836 836 676 676 676 676 676 676 676 676 676 676 676 621 621 621 621 621 621 621 621 621 621 621 836 836 836 836 836 836 836 836 836 836 463 463 463 463 463 463 463 463 463 463 463 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518

0.031 0.073 0.114 0.155 0.197 0.238 0.280 0.321 0.363 0.384 0.008 0.059 0.110 0.162 0.213 0.264 0.315 0.367 0.418 0.469 0.496 0.013 0.068 0.124 0.180 0.236 0.292 0.348 0.404 0.460 0.516 0.545 0.023 0.064 0.106 0.147 0.189 0.230 0.272 0.313 0.355 0.376 0.071 0.146 0.221 0.296 0.371 0.446 0.521 0.596 0.671 0.746 0.786 0.041 0.109 0.177 0.244 0.312 0.380 0.448 0.516 0.584 0.652 0.687 0.022 0.079 0.136 0.194 0.251 0.309 0.366 0.423 0.481

14.638 14.261 13.988 13.515 13.146 12.480 12.217 12.074 11.951 11.869 14.401 13.972 13.553 13.185 12.475 12.379 12.293 12.278 12.192 12.087 11.980 14.816 14.298 13.856 13.426 12.693 12.513 12.416 12.329 12.232 12.064 11.960 14.901 14.521 14.247 13.768 13.396 12.721 12.456 12.314 12.192 12.110 14.645 13.865 13.099 12.664 12.413 12.265 12.067 11.920 11.794 11.639 9.860 14.360 13.480 12.920 12.640 12.340 12.190 12.080 11.990 11.889 11.780 11.700 12.230 11.730 11.640 11.550 11.470 11.390 11.330 11.270 11.240

184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256

0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675

30.0 30.0 30.0 30.0 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 21.1 21.1 21.1 21.1 21.1 21.1 21.1 21.1 21.1

400 400 400 400 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518 370 370 370 370 370 370 370 370 370 370 370 370 370 370 370 370 676 676 676 676 676 676 676 676 676 676 676 676 676 676 527 527 527 527 527 527 527 527 527 527 527 527 527 527 527 518 518 518 518 518 518 518 518 518

0.685 0.737 0.789 0.816 0.012 0.054 0.097 0.130 0.164 0.198 0.232 0.266 0.300 0.334 0.368 0.401 0.435 0.469 0.487 0.011 0.070 0.129 0.189 0.236 0.284 0.331 0.379 0.426 0.474 0.521 0.569 0.616 0.664 0.711 0.736 0.014 0.047 0.073 0.098 0.124 0.150 0.176 0.202 0.228 0.254 0.280 0.306 0.332 0.345 0.012 0.053 0.095 0.128 0.162 0.195 0.228 0.262 0.295 0.328 0.362 0.395 0.428 0.462 0.479 0.026 0.071 0.096 0.125 0.153 0.181 0.210 0.238 0.267

8.330 8.294 7.347 6.210 7.370 7.200 7.160 7.240 7.310 7.280 7.360 7.390 7.430 7.380 7.440 7.410 7.480 7.470 7.510 7.574 7.519 7.290 7.306 7.355 7.384 7.383 7.452 7.470 7.387 7.486 7.484 7.511 7.449 7.527 7.530 7.380 7.360 7.330 7.320 7.350 7.400 7.390 7.410 7.450 7.420 7.480 7.470 7.490 7.530 7.370 7.200 7.160 7.240 7.310 7.280 7.360 7.390 7.430 7.380 7.440 7.410 7.480 7.470 7.510 6.500 6.470 6.490 6.510 6.410 6.580 6.670 6.530 6.780

Journal of Chemical & Engineering Data, Vol. 54, No. 9, 2009 2641 Table 1. Continued p

q

G

p

q

G

no.

MPa

kW · m-2

kg · m-2 · s-1

x

kW · m-2 · K-1

h no.

MPa

kW · m-2

kg · m-2 · s-1

x

kW · m-2 · K-1

h

74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147

0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675

42.9 42.9 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 35.1 30.0 30.0 30.0 30.0 30.0

518 518 518 518 518 518 518 518 518 518 518 518 518 676 676 676 676 676 676 676 676 676 676 436 436 436 436 436 436 436 436 436 436 436 365 365 365 365 365 365 365 365 365 365 365 333 333 333 333 333 333 333 333 333 333 333 333 321 321 321 321 321 321 321 321 321 321 321 321 676 676 676 676 676

0.538 0.568 0.001 0.048 0.095 0.142 0.189 0.236 0.282 0.329 0.376 0.423 0.448 0.012 0.048 0.084 0.120 0.156 0.192 0.228 0.264 0.300 0.319 0.031 0.087 0.142 0.198 0.254 0.310 0.366 0.422 0.478 0.534 0.563 0.057 0.123 0.190 0.257 0.324 0.390 0.457 0.524 0.591 0.657 0.692 0.002 0.075 0.148 0.221 0.294 0.367 0.440 0.513 0.586 0.659 0.732 0.771 0.006 0.082 0.158 0.233 0.309 0.385 0.461 0.537 0.613 0.689 0.765 0.805 0.031 0.069 0.100 0.130 0.161

11.180 11.130 10.500 10.370 10.250 10.160 10.110 10.070 10.020 9.970 9.920 9.870 9.840 10.179 10.135 10.111 9.988 10.012 10.007 10.012 10.007 9.992 9.970 10.473 10.330 10.188 10.125 10.063 9.991 9.928 9.876 9.824 9.761 9.710 10.201 10.055 9.949 9.920 9.862 9.824 9.755 9.697 9.648 9.619 9.580 10.504 10.347 10.166 10.096 10.005 9.924 9.853 9.773 9.702 9.621 9.540 8.310 10.479 10.323 10.134 10.065 9.986 9.886 9.807 9.718 9.638 9.549 8.820 7.330 8.340 8.270 8.210 8.230 8.260

257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330

0.675 0.675 0.675 0.675 0.675 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.696 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.701 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.705 0.712 0.712 0.712 0.712 0.712 0.712 0.712 0.712 0.712 0.712 0.712 0.714 0.714 0.714 0.714 0.714 0.714

21.1 21.1 21.1 21.1 21.1 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0

518 518 518 518 518 836 836 836 836 836 836 836 836 836 836 463 463 463 463 463 463 463 463 463 463 463 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 370 370 370 370 370 370 370 370 370 370 370 370 370 370 370 463 463 463 463 463 463 463 463 463 463 463 836 836 836 836 836 836

0.295 0.323 0.352 0.380 0.395 0.017 0.058 0.100 0.142 0.184 0.226 0.267 0.309 0.351 0.373 0.061 0.137 0.213 0.289 0.365 0.441 0.518 0.594 0.670 0.746 0.786 0.009 0.074 0.139 0.204 0.256 0.308 0.360 0.412 0.464 0.516 0.568 0.620 0.672 0.724 0.776 0.803 0.055 0.113 0.172 0.219 0.266 0.312 0.359 0.406 0.453 0.500 0.546 0.593 0.640 0.687 0.711 0.051 0.126 0.202 0.277 0.352 0.428 0.503 0.578 0.654 0.729 0.769 0.012 0.053 0.095 0.137 0.179 0.221

6.720 6.850 6.870 7.030 7.060 15.122 14.739 14.463 13.980 13.604 12.921 12.654 12.512 12.391 12.308 14.935 14.142 13.363 12.920 12.667 12.518 12.318 12.170 12.043 11.886 10.071 8.734 8.663 8.634 8.636 8.558 8.595 8.549 8.544 8.467 8.504 8.416 8.473 8.489 8.453 7.488 6.330 7.721 7.664 7.431 7.450 7.500 7.528 7.526 7.596 7.614 7.529 7.629 7.626 7.654 7.590 7.675 15.184 14.377 13.584 13.134 12.876 12.724 12.520 12.369 12.239 12.079 10.234 15.317 14.930 14.652 14.163 13.784 13.092

2642

Journal of Chemical & Engineering Data, Vol. 54, No. 9, 2009

Table 1. Continued p

q

G

p

q

G

no.

MPa

kW · m-2

kg · m-2 · s-1

x

kW · m-2 · K-1

h no.

MPa

kW · m-2

kg · m-2 · s-1

x

kW · m-2 · K-1

148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183

0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675 0.675

30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0

676 676 676 676 676 676 676 676 676 518 518 518 518 518 518 518 518 518 518 518 518 518 518 518 400 400 400 400 400 400 400 400 400 400 400 400

0.191 0.222 0.252 0.283 0.313 0.344 0.375 0.405 0.421 0.038 0.088 0.139 0.179 0.219 0.259 0.299 0.340 0.380 0.420 0.460 0.500 0.541 0.581 0.602 0.021 0.086 0.151 0.216 0.268 0.320 0.372 0.424 0.477 0.529 0.581 0.633

8.190 8.290 8.280 8.270 8.290 8.210 8.280 8.270 8.280 8.330 8.220 8.290 8.200 8.210 8.270 8.230 8.260 8.290 8.310 8.190 8.240 8.260 8.210 8.250 8.578 8.508 8.480 8.481 8.405 8.440 8.394 8.389 8.312 8.347 8.261 8.316

331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365

0.714 0.714 0.714 0.714 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.720 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727 0.727

50.0 50.0 50.0 50.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4 25.4

836 836 836 836 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 370 370 370 370 370 370 370 370 370 370 370 370 370 370 370

0.263 0.304 0.346 0.368 0.001 0.066 0.131 0.196 0.248 0.299 0.351 0.403 0.455 0.507 0.559 0.610 0.662 0.714 0.766 0.793 0.050 0.109 0.169 0.216 0.264 0.311 0.359 0.406 0.454 0.501 0.549 0.596 0.644 0.691 0.716

12.823 12.680 12.558 12.475 8.857 8.784 8.754 8.756 8.677 8.714 8.666 8.661 8.583 8.619 8.530 8.587 8.602 8.566 7.588 6.414 7.853 7.794 7.555 7.574 7.624 7.653 7.650 7.721 7.739 7.652 7.753 7.750 7.778 7.712 7.798

4. Results and Discussion Local flow boiling heat transfer coefficients are investigated over a range of vapor qualities up to 0.8. The mass flux varies from (310 to 860) kg · m-2 · s-1. The heat flux ranges from (21 to 50) kW · m-2, and the saturation pressure changes from (0.65 to 0.75) MPa. The heat transfer coefficient measured falls in the range of (6 to 15.5) kW · m-2 · K-1. The measured data are given in Table 1. Figures 3, 4, 5, and 6 show the dependences of the heat transfer coefficient on vapor quality under different saturation

Figure 3. Dependences of the heat transfer coefficient h on vapor quality x under different saturation pressures at q ) 50.0 kW · m-2 and G ) 836 kg · m-2 · s-1. Both the original and the recalculated heat transfer coefficients are drawn. The solid symbols represent the original data, and the open symbols represent the corrected data. 2 and 4, p ) 0.651 MPa; [ and ], p ) 0.675 MPa; f and g, p ) 0.696 MPa; 9 and 0, p ) 0.714 MPa.

h

pressures (average pressures of the inlet and the outlet) and heat fluxes. The heat transfer coefficients decrease with increasing vapor quality except those in the low heat and mass flux conditions. Because there is always a large pressure drop in mini-channel flow boiling experiments, especially in the high heat and mass flux conditions, it is unclear that how severe the pressure drop contributes to the heat transfer coefficient decrease. For this reason, we correlate more than 500 experimental data in this study, and the results show a moderate dependence of heat transfer coefficient on saturation pressure, namely, h ∝ p0.43. Although we are not able to confirm if this

Figure 4. Dependences of the heat transfer coefficient h on vapor quality x under different saturation pressures at q ) 50.0 kW · m-2 and G ) 463 kg · m-2 · s-1. The solid symbols represent the original data, and the open symbols represent the corrected data. 2 and 4, p ) 0.675 MPa; [ and ], p ) 0.694 MPa; f and g, p ) 0.712 MPa.

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Figure 5. Dependences of the heat transfer coefficient h on vapor quality x under different saturation pressures at q ) 30.0 kW · m-2 and G ) 400 kg · m-2 · s-1. The solid symbols represent the original data, and the open symbols represent the corrected data. 2 and 4, p ) 0.675 MPa; [ and ], p ) 0.701 MPa; f and g, p ) 0.720 MPa.

Figure 6. Dependences of the heat transfer coefficient h on vapor quality x under different saturation pressures at q ) 25.4 kW · m-2 and G ) 370 kg · m-2 · s-1. Both the original and the recalculated heat transfer coefficients are drawn. The solid symbols represent the original data, and the open symbols represent the corrected data. 2 and 4, p ) 0.675 MPa; [ and ], p ) 0.705 MPa; f and g, p ) 0.727 MPa.

dependence comes from a boiling suppression because of the high pressure in this study, we try to eliminate or at least weaken the effect of the large pressure drop by dividing the measured local heat transfer coefficients by a factor of (p/po)0.43, where po is the pressure at the outlet of each experimental run. Both the original and the recalculated heat transfer coefficients are drawn in Figures 3, 4, 5, and 6 where the solid symbols represent the original data and the open symbols represent the corrected data. Though the effect of the large pressure drop cannot be completely eliminated by this method, it can be concluded that the pressure drop is obviously not the only factor which contributes to the decrease of the heat transfer coefficient with increasing vapor quality. The dependences of the heat transfer coefficient on vapor quality under different heat and mass fluxes are depicted in Figures 7, 8, 9, and 10. As are shown in the figures, the dependences of the heat transfer coefficient on mass flux vary with different heat fluxes. In high heat flux conditions, the heat transfer coefficients are almost independent of mass flux. While at a lower heat flux of 25.4 kW · m-2, the heat transfer coefficient shows observable dependence on mass flux. Similarly, the dependences of the heat transfer coefficient on vapor quality also vary with different heat fluxes. In high heat flux conditions, the heat transfer coefficient decreases quickly with increasing vapor quality, as shown in Figures 7 and 8. When the heat flux decreases, this decreasing trend gets weaker. The heat transfer

Figure 7. Dependence of the heat transfer coefficient h on vapor quality x under different mass fluxes at q ) 50.0 kW · m-2 and an average saturation pressure of 0.675 MPa. 4, G ) 836 kg · m-2 · s-1; 0, G ) 676 kg · m-2 · s-1; ], G ) 621 kg · m-2 · s-1; g, G ) 463 kg · m-2 · s-1.

Figure 8. Dependence of the heat transfer coefficient h on vapor quality x under different mass fluxes at q ) 35.1 kW · m-2 and an average saturation pressure of 0.675 MPa. 0, G ) 676 kg · m-2 · s-1; 3, G ) 436 kg · m-2 · s-1; ], G ) 365 kg · m-2 · s-1; g, G ) 333 kg · m-2 · s-1; O, G ) 321 kg · m-2 · s-1.

Figure 9. Dependence of the heat transfer coefficient h on vapor quality x under different mass fluxes at q ) 30.0 kW · m-2 and an average saturation pressure of 0.675 MPa. 0, G ) 676 kg · m-2 · s-1; 4, G ) 518 kg · m-2 · s-1; g, G ) 400 kg · m-2 · s-1.

coefficient even increases with vapor quality when the heat flux is reduced to a low heat flux of 25.4 kW · m-2. It appears in the figures that the heat flux of 30.0 kW · m-2 is a turning point in the current experiment. Near the turning point, the heat transfer coefficient is insensitive to the vapor quality. In addition, for many experimental runs, the heat transfer coefficient decreases sharply at a vapor quality of about 0.75. Figure 11 plots the dependence of the heat transfer coefficient on vapor quality under different heat fluxes but the same mass flux. As shown in the figure, the heat transfer coefficient always

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Figure 12. Schematic drawing of the three-zone heat transfer model.

Figure 10. Dependence of the heat transfer coefficient h on vapor quality x under different mass fluxes at q ) 25.4 kW · m-2 and an average saturation pressure of 0.675 MPa. 0, G ) 676 kg · m-2 · s-1; 4, G ) 527 kg · m-2 · s-1; g, G ) 370 kg · m-2 · s-1.

Figure 11. Dependence of the heat transfer coefficient h on vapor quality x under different heat fluxes at G ) 518 kg · m-2 · s-1. 0, q ) 50.0 kW · m-2; 4, q ) 42.9 kW · m-2; ], q ) 35.1 kW · m-2; 3, q ) 30.0 kW · m-2; g, q ) 25.4 kW · m-2; O, q ) 21.1 kW · m-2.

increases with increasing heat flux, which is identified with the nucleate boiling dominant heat transfer mechanism. It is often accepted that saturated flow boiling heat transfer is governed by two mechanisms: nucleate boiling and convective boiling. For saturated flow boiling heat transfer in a mini channel, nucleate boiling and convective boiling are also thought to be the governing mechanisms.15 Therefore, the methods based on these two mechanisms are frequently applied to analyze the mini-channel experiments. Some experiments3,5,16,18 demonstrated that the flow boiling heat transfer coefficient strongly depended on heat flux and saturation pressure while hardly on mass flux and vapor quality, up to vapor qualities high as (60 to 70) %. The authors claimed that nucleate boiling dominates the heat transfer. In contrast, some other experiments17,19 showed that the heat transfer coefficient increased with the increasing vapor quality and mass flux except in a small region with low vapor qualities of (0 to 20) %. The results were explained in the forced convective dominated boiling regime. Still, some researchers9,10 found that the heat transfer coefficient started to decrease with increasing vapor quality at the very beginning of a saturated boiling when the heat flux reached a certain level. It appears that the conventional method, with only nucleate boiling and convective boiling contributions, is insufficient to explain the mini-channel flow boiling experiments. This difficulty will be partially solved if the helpful conception of periodic dry-out is introduced. This conception was proposed by Thome at al.20 In Thome’s model, bubbles are assumed to nucleate and quickly grow to the channel’s size such that

successive elongated bubbles are formed, confined radially by the tube wall, and grow in length, trapping a thin film of liquid between the bubble and the inner tube wall. Then, dry-out promptly occurs at the tail of the liquid film at the onset of saturated boiling. On the basis of this concept, a three-zone heat transfer model, which includes liquid film, liquid slug, and dryout region are established and schematically shown in Figure 12. For explanation convenience, the time-average heat transfer coefficients of dry-out, liquid film, and liquid slug regions are defined as hg, hf, and hl, respectively. In the current experiment, with a horizontal stainless steel mini tube of 1.3 mm inner diameter, dry-out may appear at the very onset of saturation boiling when the heat flux is higher than 30.0 kW · m-2. When the vapor quality increases, the period of dry-out becomes longer, and the proportion of hg increases. Meanwhile, the weights of hf and hl decrease, which explains the decreasing trend of heat transfer coefficient with vapor quality as shown in Figures 7, 8, and 11. In addition, since hf denotes the thin liquid film evaporation heat transfer coefficient and is the main contributor of the heat transfer coefficient in this region, the contribution of hl to the whole heat transfer is comparatively small. Consequently, the heat transfer coefficient shows almost independence upon mass flux. When the heat flux is around 30 kW · m-2, traditional bubbly flow is probably suited for the region of low vapor quality. In the region of high vapor quality, the three-zone model is appropriate, but the hf may not be as high as in the high heat flux region, so the increase of hg or hl may balance the decrease resulting from the increasing period of dry-out. Therefore, the heat transfer coefficient is less sensitive to the vapor quality as is shown in Figures 9 and 11. When the heat flux is less than 30.0 kW · m-2, dry-out may not exist at all. The three-zone heat transfer model should be degraded to a two-zone model, which only includes liquid film and liquid slug, as shown in Figure 12a,b. As a consequence, the convective evaporation is the main contributor of the flow boiling heat transfer, and the heat transfer coefficient increases with increasing mass flux and vapor quality. In addition, a sharp decrease of the heat transfer coefficient is also observed at a vapor quality of about 0.75. This point is likely the flow pattern transition from annular flow to annularmist flow.

5. Conclusions According to the experimental results and analysis, the following conclusions can be drawn: 1. The heat transfer coefficient always increases with the increase of saturation pressure and heat flux. 2. A turning value of the heat flux, approximately 30.0 kW · m-2, is observed in our experiments. When the heat flux is lower than this turning value, the heat transfer coefficient increases with increasing mass flux and vapor quality. When the heat flux is larger than the turning value, the heat transfer

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coefficient decreases with increasing vapor quality but is insensitive to the mass flux. Near the turning heat flux, the heat transfer coefficient is insensitive to both the mass flux and the vapor quality. 3. Dry-out is an important concept in mini-channel flow boiling. The heat transfer behavior depends on the competition between the decrease of the heat transfer coefficient resulting from the increase of dry-out and the increase of the heat transfer coefficient contributed by the thin liquid film evaporation.

Literature Cited (1) Wadekar, V. V. Flow boiling of heptane in a plate-fin heat exchangers passage, Compact Heat Exchangers for Power and Process Industries, HTD. Proc. ASME Heat Transfer DiV. 1993, 201, 1–6. (2) Carey, V. P. Two-phase flow in small-scale ribbed and finned passages for compact evaporators and condensers. Nucl. Eng. Des. 1993, 141, 249–268. (3) Kew, P. A.; Cornwell, K. Correlations for the prediction of boiling heat transfer in small-diameter channels. Appl. Therm. Eng. 1997, 17, 705–715. (4) Wilmarth, T.; Ishii, M. Two-phase flow regimes in narrow rectangular vertical and horizontal channels. Int. J. Heat Mass Transfer 1994, 37, 1749–1758. (5) Wambsganss, M. W.; France, D. M.; Jendrzejczyk, J. A.; Tran, T. N. Boiling heat transfer in a small-diameter tube. J. Heat Transfer 1993, 115, 963–972. (6) Oh, H.; Katsuta, M.; Shibata, K. Heat transfer characteristics of R134a in a capillary tube heat exchanger. Proc. Int. Heat Transfer Conf. 11th 1998, 6, 131–136. (7) Zhang, W.; Hibiki, T.; Mishima, K. Correlation for flow boiling heat transfer at low liquid Reynolds number in small diameter channels. J. Heat Transfer 2005, 127, 1214–1221. (8) Bertsch, S. S.; Groll, E. A.; Garimella, S. V. Effect of heat flux, mass flux, and saturation temperature on flow boiling heat transfer in microchannels. Int. J. Multiphase Flow 2009, 35, 142–154.

(9) Lin, S.; Kew, P. A.; Cornwell, K. Two-phase heat transfer to a refrigerant in a 1 mm diameter tube. Int. J. Refrig. 2001, 24, 51–56. (10) Huo, X.; Chen, L.; Tian, Y. S.; Karayiannis, T. G. Flow boiling and flow regimes in small diameter tubes. Appl. Therm. Eng. 2004, 24, 1225–1239. (11) Shiferaw, D.; Karayiannis, T. G.; Kenning, D. B. R. Flow boiling in a 1.1 mm tube with R134a: Experimental results and comparison with model. Int. J. Therm. Sci. 2009, 48, 331–341. (12) ASHRAE Handbook Fundamentals 2005; American Society of Heating, Refrigerating, and Air-Conditioning Engineers: Atlanta, GA, 2005. (13) Tran, T. N.; Wambsganss, M. W.; France, D. M. Small circular and rectangular channel boiling with two refrigerants. Int. J. Multiphase Flow 1996, 22, 485–498. (14) Moffat, R. J. Describing the uncertainties in the experimental results. Exp. Therm. Fluid Sci. 1988, 1, 3–17. (15) Kandlikar, S. G. Heat Transfer Mechanisms During Flow Boiling in Microchannels. J. Heat Transfer 2004, 126, 8–16. (16) Bao, Z. Y.; Fletcher, D. F.; Haynes, B. S. Flow boiling heat transfer of R11 and R123 in narrow passages. Int. J. Heat Mass Transfer 2001, 43, 3347–3358. (17) Lazarek, G. M.; Black, S. H. Evaporative heat transfer, pressure drop and critical heat flux in a small diameter vertical tube with R113. Int. J. Heat Mass Transfer 1982, 25, 945–960. (18) Vlasie, C.; Macchi, H.; Guilpart, J.; Agostini, B. Flow boiling in small channels. Int. J. Refrig. 2004, 27, 191–201. (19) Kureta, M.; Kobayashi, T.; Mishima, K.; Nshihara, H. Pressure drop and heat transfer for flow boiling of water in small-diameter tubes. JSME Int. J., Ser. B 1998, 41, 871–879. (20) Thome, J. R.; Dupont, V.; Jacobi, A. M. Heat transfer model for evaporation in microchannels. Int. J. Heat Mass Transfer 2004, 47, 3375–3385. Received for review January 30, 2009. Accepted July 6, 2009. We thank the Scientific Research Foundation for the Returned Overseas Chinese Scholars by the Ministry of Education in China and the Alexander von Humboldt Foundation in Germany for financial support.

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